1. Field of the Invention
The invention relates to an optical touch device and a method for calculating coordinate of touch point and, more particularly, to an optical touch device and a method for calculating coordinate of touch point capable of preventing a touch trajectory from shifting.
2. Description of the Prior Art
Since consumer electronic products have become more and more lighter, thinner, shorter, and smaller, there is no space on these products for containing a conventional input device, such as mouse, keyboard, etc. With development of touch technology, in various kinds of consumer electronic products (e.g. display device, all-in-one device, mobile phone, personal digital assistant (PDA), etc.), a touch device has become a main tool for data input. Compared with other touch design, such as a resistive touch design, a capacitive touch design, an ultrasonic touch design, or a projective touch design, an optical touch design has lower cost and is easier to use, especially for large-size touch display.
A conventional optical touch device senses a touch point indicated by a touch object (e.g. finger or stylus) on an indication plane by two image sensing units arranged oppositely. When the image sensing units sense the touch object on the indication plane, a processing unit of the optical touch device can calculate the touch point indicated by the touch object accordingly. However, since the resolution of the image sensing unit is limited, the size of the optical touch device is also limited accordingly.
Referring to FIG. 1, FIG. 1 is a schematic diagram illustrating an optical touch device 5 of the prior art. As shown in FIG. 1, the optical touch device 5 disposes four image sensing units 52, 54, 56, 58 at the center of an indication plane 50, wherein the image sensing units 52, 58 are used for sensing the right touch area of the indication plane 50 and the image sensing units 54, 56 are used for sensing the left touch area of the indication plane 50. Accordingly, the optical touch device 5 may have a large size.
As shown in FIG. 1, a central touch area 500 is defined between the image sensing units 52, 54, 56, 58, wherein a left boundary 502 of the central touch area 500 connects the image sensing units 52, 58 and a right boundary 504 of the central touch area 500 connects the image sensing units 54, 56. In general, a touch operation performed on the central touch area 500 can be sensed by the image sensing units 52, 58 or the image sensing units 54, 56. As shown in FIG. 1, a touch trajectory 60 is sensed by the image sensing units 54, 56 and another touch trajectory 62 is sensed by the image sensing units 52, 58. For the image sensing units 54, 56, the touch trajectory 60 will shift while it is close to the right boundary 504. Similarly, for the image sensing units 52, 58, the touch trajectory 62 will shift while it is close to the left boundary 502. Accordingly, the transition between the touch trajectories 60 and 62 within the central touch area 500 will be uneven and this phenomenon will affect the visual effect of a user.